1. Field of the Invention
The present invention relates to a paper machine press roll and more particularly, it relates to a press roll which is used in a press section of a paper machine to remove water from a wet paper web and to transfer smoothness to a surface of the wet paper web.
2. Description of the Background Art
As a pressing method of a wet paper web in a press section of a paper machine, a roll press and an extended nip press (ENP) are well known. The roll press is a system of removing water from the wet paper web while the wet paper web is held on a felt and passes between pressurized two revolving rolls, and the extended nip press is a system of removing water from the wet paper web while the wet paper web is held on a felt and passes between a belt pressurized by a pressure shoe having a large nip width and a revolving roll.
In either system, the revolving roll has a hard surface in view of a pressurizing effect and a smoothness-transfer effect. In the roll press, the revolving roll having the hard surface and a roll covered with a rubber or the like are combined to be used.
It is expected that the revolving roll having the hard surface has performance which transfers smoothness to a surface of the paper and can withstand high loading, high-speed revolving and long use. In order to answer the above expectation, conventionally a stone roll (granite roll) using natural granite has been used. According to the stone roll, since it can provide mirror surface finish in general, its smoothness-transfer effect to the paper is excellent, its surface is hard, and wear caused by a doctor blade which is provided for removing waste paper is small. In addition, according to the stone roll, since it has concave parts in the roll surface because of voids of crystal particles or cleavages of mica contained in the granite and these provide an appropriate water holding property, a paper peeling property of the wet paper web is preferable. Furthermore, since it has a property in which pitch and the like contained in pulp is not likely to be adhered even in long-term use, the paper is not likely to be cut at the time of press.
Although the stone roll has the above advantages, it has the following defects because it is a natural stone. First, the natural stone is not easily available because of depletion resources, so that it becomes very expensive and it takes long time until the stone is delivered. In addition, it is very difficult to quarry, deliver and process a large stone for the press roll which is becoming long and large. Furthermore, since the stone is a polycrystalline natural stone, there is variation in surface characteristics (such as surface asperity, surface hardness, a water holding property and the like) depending on manufactured rolls and even on the surface of one roll, there is variation in the surface characteristics, which is an essential problem.
As a substitute for the stone roll, there is disclosed a synthetic stone roll in which powder particles of granite, quartz sand or the like are mixed in a hard rubber or a hard resin in Japanese Unexamined Patent Publication No. 50-90704. However, according to the synthetic stone roll, its water holding property on the surface is not sufficient and the paper peeling property is not good basically. In addition, since an organic matter is used as a binder, pitch is likely to be adhered and since its surface hardness is low, the uniformity on the surface is damaged by the doctor blade. Thus, the paper is likely to be cut or wrinkle up in a short time, so that the synthetic stone roll is not suitable for a long operation.
As a press roll which can solve the above problems, there is proposed a ceramics sprayed roll in which ceramics is sprayed on a core roll. As a method for forming a ceramics film on the core roll, there is a water stabilized plasma spraying method, a gas plasma spraying method or the like. A paper machine press roll in which a ceramics film is formed on a core roll by the water stabilized plasma spraying method is disclosed in Europe Patent Publication No. EP0207921B, and a paper machine press roll in which a ceramics film is formed on a core roll by the gas plasma method is disclosed in Japanese Unexamined Patent Publication No. 8-232188, for example.
In the case of the water stabilized plasma spraying method, since a particle diameter of the ceramics powder used as a sprayed material in general is large, the sprayed film is not finely formed. Therefore, it has been thought that the paper machine press roll in which the ceramics film is formed by the water stabilized plasma sprayed method has a coarse roll surface and an inferior smoothness-transfer effect to the paper, and a doctor blade attached on the roll surface is excessively worn. Therefore, the water stabilized plasma spraying method dose not attract attention as the method of forming the ceramics film of the paper machine press roll.
Meanwhile, in the case of the gas plasma spraying method, since a particle diameter of ceramics powder to be used as a sprayed material is small, a sprayed film is finely formed. In this respect, the paper machine press roll in which the ceramics film is formed by the gas plasma spraying method prevails at present.
However, according to an examination by the inventor of the present invention, in the case of the press roll in which the ceramics film is formed by the gas plasma spraying method, since the sprayed film is fine, smoothness of the roll surface is preferable but adhesiveness between the roll surface and the wet paper web is high, so that the paper peeling property deteriorates. When the paper peeling property is low, a surface of the wet paper web is adhered to the roll surface as waste paper while the wet paper web passes through a press nip, so that paper quality deteriorates. Furthermore, when the paper peeling property is low, it is necessary to increase a drawing value (a degree of drawing force required to peel the wet paper web from the roll surface) provided between the press nip and the next step. If the drawing value is great, the paper is deformed by tensile force to peel the wet paper web from the roll surface. As a result, the paper quality is lowered. Furthermore, when the paper peeling property is low, the paper could be cut between the press roll and the next step, which could cause serious damage to a paper factory. Thus, the inferior paper peeling property could be a fatal defect for the press roll.
Conventionally, according to the ceramics film of the press roll, it has been thought that its surface roughness is preferably small in view of the smoothness-transfer effect to the paper and wear prevention of the doctor blade, but the surface roughness is preferably large in view of the paper peeling property. As an index for evaluating the surface roughness, arithmetic average roughness Ra defined in JIS B0601-2001 (ISO4287-1997) has been used. The arithmetic average roughness Ra is basically found by equivalently treating a height of a convex part and a depth of a concave part as absolute values and averaging them. However, according to an examination and study by the inventor of the present invention, the surface characteristics of the paper machine press roll comprising the ceramics sprayed film cannot be correctly evaluated only by the arithmetic surface roughness Ra.
In the paper machine press roll, only the convex part of the press roll affects the smoothness-transfer effect to the paper and the wear of the doctor blade, so that the smoothness-transfer effect to the paper and the wear prevention effect of the doctor blade can be enhanced by reducing the height of the convex part. Meanwhile, the concave part of the press roll affects the paper peeling property of the wet paper web, so that the water holding property and an air acquiring property at the concave part can be improved by increasing the depth or capacity of the concave part. As a result, the adhesiveness of the wet paper web can be lowered and the paper peeling property is improved. In the case the surface roughness of the press roll is evaluated with the arithmetic average roughness Ra like in the conventional example, when the value of Ra is reduced in order to enhance the smoothness-transfer effect to the paper and to prevent the wear of the doctor blade, the paper peeling property is lowered, but when the value of the Ra is increased in order to improve the paper peeling property of the wet paper web, the smoothness-transfer effect is lowered and the doctor blade is likely to be worn. Thus, even if the arithmetic average roughness Ra of the roll surface is set at any value, the smoothness-transfer effect to the paper is lowered and the doctor blade is worn in an early stage, or the paper peeling property is lowered, so that it is difficult to provide satisfactory performance in all of the above aspects.